Can't change base color of icing
You have plugged the Color output of an Object Info node into the Base Color of the Principled BSDF. I would suggest you switch to the Shading workspace to properly see the nodes in the Shader Editor. Then unplug the incorrect node connection and adjust the Base Color to your liking.
Alternatively, if you want to only use the Material Properties instead of the Shader Editor, click on the Base Color like in your screenshot and select Remove.